In the automotive art, the term "actuating energy" is not to be understood in a strictly physical sense. Instead, actuating energy is a collective term for all media which, when supplied to a wheel brake, produce a braking force. The "wheel braking factor" is a value which is practically constant for each assembly consisting of the wheel, the wheel brake, the brake actuator and, possibly, existing transmission devices. The wheel braking factor includes all of the characteristics of the brake actuator (e.g., brake cylinder), ratios of the transmission devices, the characteristics of the wheel brake (e.g., internal ratios, internal frictional value), and the efficiencies of these devices, as well as the wheel size itself. The wheel braking factor denotes the relationship between an actuating energy supplied to a wheel brake and the resulting braking force between the appertaining wheel and the roadway. This braking force acts tangentially in the contact surface of the wheel. If the wheel braking factor is designated "A," the momentary actuating energy "E," and the resulting braking force "B," the following formula generally applies: EQU B=A.multidot.E (I)
Generally, the "actuating energy" is to be understood as being the "effective actuating energy" which remains after deduction of the response energy. The response energy is based on response resistances which are to be attributed to friction and resetting forces in the braking components.
A "reference velocity" is produced in the ABS control system in vehicles equipped with an anti-lock brake system, as is customary. The reference velocity is based on one or several wheel velocities at the beginning of a braking action and on the true course or on a predicted course of the wheel speed(s) during braking. The reference velocity represents a substitute value for the vehicle speed based upon which the ABS control system evaluates the locking tendency of the braked vehicle wheel or wheels, and controls the course of ABS regulation.
In vehicles with several braked axles or axle groups, the actuating energy distribution among the wheel brakes of the axles or axle groups must be stability-optimizing. The term "axle groups" means several axles which are located so close together or are so coordinated with each other that they act as one axle. For example, a double axle aggregate with a distance between axles of up to 1.6 m constitutes an axle group if the axles are statically and dynamically balanced among each other with respect to load distribution and utilization of frictional value between wheels and roadway.
"Stability-optimizing actuating energy distribution" means that the braking system brings about such a distribution of the actuating energy among the wheel brakes that in braking actions which are not ABS-regulated, the same frictional value utilization takes place between all wheels and the roadway. The consequence of such an actuating energy distribution is an optimal vehicle deceleration with good directional stability. The braking system can bring about such an actuating energy distribution completely or nearly completely. Conventional braking system are usually provided with a load-dependent braking force regulator for the purpose of generating this type of actuating energy distribution. In the art, this type of system is designated by the abbreviation "ALB," while the braking systems with electrical control, hereinafter called "EBS," achieve this distribution normally through the suitable design of an EBS control device. Further details in this matter and indications for obtaining such an actuating energy distribution in vehicles with EBS are found, for example, in EP 0 548 488 A1 and in U.S. Pat. No. 5,338,106 which are both based on the same German parent application.
A process is known from EP 0 453 811 A2, specifically from the bottom of page 5 to the top of page 6 of that patent document. The process described therein determines the available and therewith the utilized frictional value between a wheel and the roadway in an ABS-regulated braking action as the wheel starts to roll back up, i.e., from the wheel acceleration once a minimum wheel velocity has been reached. The "available frictional value" refers to the highest frictional value that the roadway can offer which value varies with the road surface conditions, e.g., an asphalt roadway will offer an available frictional value of up to .mu.=0.9 when dry and perhaps .mu.=0.1 when icy. This means that on a dry road, a brake force of 90% of the wheel load can be transferred between wheel (tire) and roadway, while on an icy road, only a brake force of 10% of the wheel load can be transferred. The "utilized frictional value" is the fraction of the available frictional value actually utilized during a braking action. In braking actions on a dry road surface, except when wheel lock occurs, the utilized frictional value is rarely equal to the available frictional value.